Microstructures of Metamorphic Rocks

Isograds and chemical reactions in regional metamorphism

Metamorphism involves the growth of new minerals in response to
changes in temperature and pressure. More than a hundred years ago,
in this classic area of the Scottish Highlands, George Barrow mapped
a series of zones representing increasing grade of metamorphism,
each characterised by a particular index mineral not found in
the preceding zone. In the 1920's, C. E. Tilley recognised that an
isograd, the line on the ground marking the first appearance
of an index mineral, reflects a chemical reaction which took place at
specific temperature and pressure conditions within the earth.

For example, the garnet isograd in the Scottish Highlands results
from the reaction

chlorite + muscovite + quartz = garnet + biotite +
H2O

Chemically, this can be expressed as

(Mg,Fe)5Al2Si3O10(OH)8
+ KAl3Si3O10(OH)2 +
SiO2 =

(Fe,Mg)3Al2Si3O12
+ K(Fe,Mg)3AlSi3O10(OH)2
+ H2O

Whereas the original sedimentary rocks are fine grained, the new
minerals formed in these reactions tend to form much larger isolated
crystals, or porphyroblasts.

Garnet porphyroblasts 1 - 2 mm in diameter in
mica schist from the garnet zone of the Scottish Highlands.